Aristotle's thesis in "Physics" I 8 is that a certain old and familiar problem about coming to be can only be solved with the help of the new account of the "principles" he has developed in "Physics" I 7. This is a strong thesis and the literature on the chapter does not quite do it justice; specifically, as things now stand we are left wondering why Aristotle should have found this problem so compelling in the first place. In this paper (...) I develop an interpretation which (I hope) will help to remedy this. I believe that Aristotle's problem about coming to be depends on a certain principle to the effect that "nothing can become what it already is" (it is this that is supposed to explain why τὸ ὄν cannot come to be ἐξ ὄντος -- cf. 191a30). The main innovation of the interpretation developed here is its suggestion that we understand this principle as a principle about kinds. So understood, the principle does not make the comparatively trivial point that nothing can become any individual it already is, but rather the more powerful and substantive point that nothing can become any kind of thing it already is. I argue that this is a point which Aristotle himself accepts and that this is why the problem about coming to be raises serious difficulties for him. I also discuss Aristotle's proposed solutions to this problem, explaining how each draws on his new account of the principles and why each is required for any full resolution of the difficulties the problem raises. In this way I hope to show how the interpretation developed here does justice to the very strong thesis with which Aristotle begins "Physics" I 8. I conclude briefly and somewhat speculatively with a suggestion as to why Aristotle might accept the principle on which I have suggested the problem turns. (shrink)